1. Field of the Invention
This invention relates to inkjet printing apparatus and methods for inkjet printing using ink that is curable upon exposure to actinic radiation. More particularly, the present invention is directed to methods and apparatus for curing radiation curable ink that has been applied to a substrate by an inkjet printer.
2. Description of the Related Art
Inkjet printing has increased in popularity in recent years due to its relatively high speed and excellent image resolution. Moreover, inkjet printing apparatus used in conjunction with a computer provides great flexibility in design and layout of the final image. The increased popularity of inkjet printing and the efficiencies in use have made inkjet printing an affordable alternative to previously known methods of printing.
In general, there are three types of inkjet printers in widespread use: the flat bed printer, the roll-to-roll printer and the drum printer. In the flat bed printer, the medium or substrate to receive the printed image rests on a horizontally extending flat table or bed. An inkjet print head is mounted on a movable carriage or other type of mechanism that enables the print head to be moved along two mutually perpendicular paths across the bed. The print head is connected to a computer that is programmed to energize certain nozzles of the print head as the print head traverses across the substrate, optionally using inks of different colors. The ink on the substrate is then cured as needed to provide the desired final image.
In roll-to-roll inkjet printers, the substrate to receive the printed image is commonly provided in the form of an elongated web or sheet and advances from a supply roll to a take-up roll. At a location between the supply roll and the take-up roll, a print head is mounted on a carriage that is movable to shift the print head across the substrate in a direction perpendicular to the direction of advancement of the substrate. Known roll-to-roll inkjet printers include vertical printers, wherein the substrate moves in an upwardly direction past the print head, as well as horizontal printers, wherein the substrate moves in a horizontal direction past the print head.
Drum inkjet printers typically include a cylindrical drum that is mounted for rotational movement about a horizontal axis. The substrate is placed over the periphery of the drum and an inkjet print head is operable to direct dots or drops of ink toward the substrate on the drum. In some instances, the print head is stationary and extends along substantially the entire length of the drum in a horizontal direction. In other instances, the length of the print head is somewhat shorter than the length of the drum and is mounted on a carriage for movement in a horizontal direction across the substrate.
Inks that are commonly used in inkjet printers include water-based inks, solvent-based inks and radiation-curable inks. Water-based inks are used with porous substrates or substrates that have a special receptor coating to absorb the water. In general, water-based inks are not satisfactory when used for printing on non-coated, non-porous films.
Solvent-based inks used in inkjet printers are suitable for printing on non-porous films and overcome the problem noted above relating to water-based ink. Unfortunately, many solvent-based inks contain about 90 percent organic solvents by weight. As solvent-based inks dry, the solvent evaporates and may present an environmental hazard. Although environmental systems may be available for reducing the emission of solvents to the atmosphere, such systems are generally considered expensive, especially for the owner of a small print shop.
Furthermore, inkjet printers using either solvent-based inks or water-based inks must dry relatively large quantities of solvent or water before the process is considered complete and the resulting printed product can be conveniently handled. The step of drying the solvents or water by evaporation is relatively time-consuming and can be a rate limiting step for the entire printing process.
In view of the problems noted above, radiation-curable inks have become widely considered in recent years as the ink of choice for printing on a wide variety of non-coated, non-porous substrates. The use of radiation curing enables the ink to quickly dry (commonly considered as xe2x80x9cinstantxe2x80x9d drying) without the need to drive off large quantities of water or solvent. As a result, radiation curable inks can be used in high speed inkjet printers that can achieve production speeds of over 1000 ft2/hr (93 m2/hr.) Inkjet printers that are capable of printing on relatively large substrates are considered expensive. Accordingly, it is desired to use the same printer to impart images to a wide variety of substrates using a wide variety of ink compositions if at all possible. Moreover, it is preferred that each image printed by such printers be of high quality on a consistent basis regardless of the type of substrate and the type of ink used, in view of the time and expense of reprinting the image in instances where the quality of the image is less than desired.
There are a wide variety of curing devices available for hardening radiation curable ink after the ink has been applied to the substrate. For example, ultraviolet (xe2x80x9cUVxe2x80x9d) lamps are often used to cure inks that are curable upon exposure to ultraviolet radiation. However, many lamps that emit ultraviolet radiation also emit significant quantities of heat during operation.
Unfortunately, the presence of excess heat can adversely affect some substrates used in inkjet printing. For example, certain substrates that are relatively thin, such as plasticized cast vinyl films, may begin to soften or melt in the presence of heat from an ultraviolet curing device. It is possible to modify the UV lamp, for example by adding an infrared filter, to reduce the amount of heat reaching the substrate, although such modifications add to the capital cost and may adversely affect the compact design of typical UV lamps used in inkjet devices.
Moreover, many attempts have been made in the past to convert conventional inkjet printers such as inkjet printers using solvent-based ink to inkjet printers that use radiation-curable ink. The cost of such a conversion is not inexpensive but is typically considerably less than the cost of buying a new printer that is specifically manufactured for use with radiation curable inks. The conversion is often carried out by mounting a source of radiation within the cabinet of the printer.
However, space that is available within the cabinets of existing printers is usually limited. Consequently, the radiation source is often mounted by necessity in close proximity to the location of the substrate during a curing operation. The resultant close spacing between the source of radiation and the substrate is often too small to permit the use of certain types of substrates (such as the substrates mentioned above) that might otherwise begin to soften in the presence of heat from the radiation source.
In view of the foregoing, there is a need in the art for new methods and apparatus for curing radiation curable ink used in an inkjet printing process. Preferably, such a method and apparatus could be used for retrofitting conventional inkjet printers as well as for constructing new printers, and could be used in conjunction with a wide variety of substrates and inks.
The present invention is directed to inkjet printing apparatus and methods for inkjet printing that employ a shield along with a mechanism for selective movement of the shield. The shield is movable into and out of a path of radiation that extends from a curing device (such as a lamp) to the substrate. The shield enables the amount of radiation reaching the substrate to be precisely controlled so that the likelihood of overheating the substrate is reduced.
In more detail, the present invention is directed in one aspect toward inkjet printing apparatus for radiation curable ink. The apparatus includes a support for receiving a substrate and a print head for directing radiation curable ink onto the substrate. The apparatus also includes a curing device for directing radiation along a path toward ink received on the substrate, and the print head is movable relative to the curing device. The apparatus further includes a shield and a mechanism for selectively moving the shield into and out of the path as may be desired in order to hinder the passage of radiation to ink received on the substrate.
The present invention is directed in another aspect to a method of inkjet printing. The method includes the act of applying a quantity of ink to a substrate using a print head and the act of directing actinic radiation from a curing device along a path toward the ink on the substrate, wherein the print head is movable relative to the curing device. The method further includes the act of selectively moving a shield into and out of the path as may be desired in order to hinder the passage of actinic radiation to the ink on the substrate.
In one embodiment of the invention, the shield comprises a rotatable, cylindrical housing having an elongated opening for selective passage of radiation. In another embodiment of the invention, the shield comprises a reciprocating shutter that is movable between an open and a closed position in order to control the passage of actinic radiation. Preferably, movement of the shield is controlled in accordance with an operational speed of the printer, such as speed of movement of the substrate past the curing device.